This invention relates to the synthesis of hydrocarbons from a methane source and, more particularly, to a method for converting natural gas to gasoline-range hydrocarbons.
The composition of natural gas at the wellhead varies quite widely, but the major hydrocarbon present is methane. For example, the methane content of natural gas may vary within the range from about 40 to about 95 volume %. Other constituents of natural gas include ethane, propane, butane, pentane, hydrogen sulfide, carbon dioxide, helium and nitrogen.
Two of the typical methods normally used to synthesize gasoline from natural gas require the initial step of steam reforming to produce synthesis gas, i.e. a mixture of CO and H.sub.2. In the Fischer-Tropsch route, the synthesis gas is reacted over an iron catalyst to give a wide spectrum of products from light gases to waxes, with gasoline comprising only a small fraction of the products. In the methanol to gasoline route, the synthesis gas is first converted to methanol, which is further converted over an acidic zeolite catalyst to produce hydrocarbons in the gasoline range.
The alkylation of methane and olefins (alkynes) has received considerable attention since Olah, U.S. Pat. No. 4,433,192 described the alkylation between methane and ethylene over a solid superacid catalyst.
The upgrading of alkane-alkene mixtures such as isobutane with isobutene or any other C.sub.4 olefin by alkylation or oligomerization to gasoline-range components, e.g. C.sub.8 alkylate, is well known in the petrochemical industry. These reactions are acid catalyzed and employ HF and H.sub.2 SO.sub.4 catalysts. Until recently, the alkylation of lower alkanes with alkenes has been studied in liquid acid media and liquid phase reaction conditions were considered to substantially limit practical application of the reaction. In addition, the products were a mixture of oily oligomers having molecular weights from about 100 to 700.
Numerous patents describe the oligomerization, alkylation and aromatization of alkanes, e.g. ethane, propane and butane and alkenes, e.g. ethene, propene and butene. Medium and large pore zeolites, e.g. ZSM-3, ZSM-5, ZSM-18, ZSM-20, faujasite, mordenite and type Y in their hydrogen and rareearth exchanged forms, are used for this purpose. For instance, conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,764 and 4,021,502, in which gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins, are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of ZSM-5 zeolite.
It is the object of the present invention to provide a simple and inexpensive two-stage method for converting natural gas to gasoline-range hydrocarbons.